Crystal chemistry and structural refinement of chromites from different chromitite layers and xenoliths of the Bushveld Complex

Crystal chemistry and structural refinement of 20 chromite crystals representative of different chromitites layers and xenoliths from the Critical Zone of the Bushveld Complex have been studied. Small differences in the structural parameters of chromites have been found along the Bushveld sequence and these allowed to discriminate two main subdivision between the Lower and Middle Group samples and the Upper Group and Merensky Reef samples. The small increase of chromite oxygen coordinates seems to be related with the decrease of cooling rates as estimated from the filling and cooling model for the Bushveld Complex proposed by Cawthorn & Walraven (1998). In fact, the cooling rates for Upper Group and Merensky Reef are smaller than that of Lower Group and Middle Group. Moreover, peculiar structural parameters have been found for chromites in the Tweefontein pipe suggesting different cooling rates.